1. Field of the Invention
The present invention generally relates to an electrolytic cell for electrolysis of an aqueous alkali metal halide solution, especially an aqueous alkali metal chloride solution. More particularly, it relates to an apparatus for mainly obtaining a high quality caustic alkali more effectively with low cell voltage using a horizontal type electrolytic cell providing a cation exchange membrane as an electrolytic separator.
2. Description of Prior Art
The horizontal type electrolytic cell is partitioned by an asbestos diaphragm positioned substantially horizontal into an upper anode compartment and a lower cathode compartment and has been in considerably widespread use industrially, because of an advantage that the product, for example, caustic alkali is produced in the cathode compartment, therefore, does not move to the anode compartment through the diaphragm.
The most typical horizontal electrolytic cell is a mercury electrolytic cell but destined to be shut down in the near future since mercury served as a cathode contaminates environment. When such a mercury cathode electrolytic cell is desired to be converted into a separator electrolytic process employing no mercury with a reduced cost, the separator electrolytic process should be of a horizontal type. In view of the situation, it is a significant matter the industry is now encountering to develop a process for producing a high quality product, not inferior to a product by the mercury process, with a high current efficiency using such horizontal type separator electrolytic cells.
A process for retrofitting a mercury cell to a horizontal type separator cell is revealed in U.S. Pat. No. 3,923,614. In the patent, however, a porous membrane (asbestos diaphragm) is used to serve as a separator, having great water permeability and accordingly anolyte solution passes through the separator hydraulically to thus mingle in, for example, caustic alkali produced in the cathode compartment, thereby resulting in decreased quality.
On the other hand, a cation exchange membrane called a nonporous membrane permits no passage of anolyte solution or catholyte liquor hydraulically, allowing only water molecules coordination-bonded to alkali metal ions transported electrically to pass, hence a high quality caustic alkali being obtained. To the contrary, a small quantity of water transported evaporates to cause electric conduction failure between a membrane and a cathode, in the long run to terminate electrolytic reaction.
U.S. Pat. No. 3,901,774 proposes processes to solve the problems; one is a process for placing a liquid maintaining material between a cation exchange membrane and a cathode and another is a process for carrying out the electrolysis while supplying into a cathode compartment an aqueous caustic alkali solution in the form of mist or spray in order to maintain electric conductivity.
Notwithstanding, the former process not only involves the problems including troubles for interposing the liquid maintaining material and the durability thereof, but increases cell voltage because the distance between electrodes is expanded by the liquid maintaining material located between the cation exchange membrane and the cathode, besides an increase in electric resistance of the liquid maintaining material per se. Hence it can not be an advantageous process. Moreover the latter process has some difficulties in practice on an industrial scale since the uniform supply of liquid is difficult when applied to a large-scale electrolytic cell such as employed commercially.
In an attempt to eliminate the foregoing defects attendant on the conventional processes, a process and apparatus therefor has been developed by the present applicant and proposed by U.S. Ser. No. 434,737 (EPC Appln. No. 82109528). This proposal involves an electrolytic process characterized in that hydrogen gas generated on a cathode is allowed to be enfolded in a stream of catholyte liquor and removed out of a cathode compartment, and electrolytic cell which is characterized by an upper anode compartment and a lower cathode compartment partitioned by a cation exchange membrane positioned substantially horizontal, said anode compartment having therein substantially horizontal anodes and being surrounded by a top cover, side walls positioned so as to enclose the anodes and the upper side of the membrane, and being provided with an inlet and an outlet of anolyte solution and an outlet of anode gas, said cathode compartment being surrounded by a cathode plate, side walls so as to enclose the cathode plate and the underside of the membrane, and being provided with an inlet of catholyte liquor and an outlet of a mixed stream of the cathode gas and the catholyte liquor.
However, during the course of further study, it has been found out by the inventors; (1) Non-uniform flow of catholyte liquor (mixed stream) and dead space occur owing to adhesion of cathode gas to the membrane. This dead space causes variation of pressure difference (.DELTA.p) in the flow of catholyte liquor (mixed stream) between catholyte liquor inlet and mixed stream outlet, brings about vibration of the membrane and damages the membrane through collision of the membrane with electrodes. (2) Maintaining of uniform anode-cathode gap is difficult. (3) When the cathode plate is not substantially flat on its surface (for instance, cathode plate having a concave-convex surface or a coarse surface) the membrane contacts with and rubs against the cathode plate to thus result in damage of the membrane. As a result, stable operation for a long period is prevented.
The present invention has been completed in order to eliminate the deficiencies attendant on the conventional processes as aforesaid and enables the retrofit of a mercury cell into a horizontal type cation exchange membrane cell with a relative ease, at the same time, achieving the production of a high quality caustic alkali with a high current efficiency. The present invention is, of course, useful in newly constructing a cell with new materials.